1. Field of the Invention
This invention relates to the field of signal detection using correlation analysis, and more specifically, to correlation analysis in which coherent integration is employed in order to more rapidly achieve a target signal to noise ratio (SNR).
2. Background of the Invention
The Global Positioning System (GPS) is a collection of 24 earth-orbiting satellites. Each of the GPS satellites travels in a precise orbit about 11,000 miles above the earth""s surface. A GPS receiver locks onto at least 3 of the satellites, and responsive, thereto, is able to determine its precise location. Each satellite transmits a signal modulated with a unique pseudo-noise (PN) code. Each PN code comprises a sequence of 1023 chips which are repeated every millisecond consistent with a chip rate of 1.023 MHz. Each satellite transmits at the same frequency. For civil applications, the frequency is known as L1 and is 1575.42 MHz. The GPS receiver receives a signal which is a mixture of the transmissions of the satellites that are visible to the receiver.
The receiver detects the transmission of a particular satellite by correlating the received signal with shifted versions of the PN code for that satellite. If the level of correlation is sufficiently high so that there is a peak in the level of correlation achieved for a particular shift and PN code, the receiver detects the transmission of the satellite corresponding to the particular PN code. The receiver then uses the shifted PN code to achieve synchronization with subsequent transmissions from the satellite.
The receiver determines its distance from the satellite by determining the code phase of the transmission from the satellite. The code phase (CP) is the delay, in terms of chips or fractions of chips, that a satellite transmission experiences as it travels the approximately 11,000 mile distance from the satellite to the receiver. The receiver determines the code phase for a particular satellite by correlating shifted versions of the satellite""s PN code with the received signal after correction for Doppler shift. The code phase for the satellite is determined to be the shift which maximizes the degree of correlation with the received signal.
The receiver converts the code phase for a satellite to a time delay. It determines the distance to the satellite by multiplying the time delay by the velocity of the transmission from the satellite. The receiver also knows the precise orbits of each of the satellites. Updates to the locations of the satellites are transmitted to the receiver by each of the satellites. This is accomplished by modulating a low frequency (50 Hz) data signal onto the PN code transmission from the satellite. The data signal encodes the positional information for the satellite. The receiver uses this information to define a sphere around the satellite at which the receiver must be located, with the radius of the sphere equal to the distance the receiver has determined from the code phase. The receiver performs this process for at least three satellites. The receiver derives its precise location from the points of intersection between the at least three spheres it has defined.
The Doppler shift (DS) is a frequency shift in the satellite transmission caused by relative movement between the satellite and the receiver along the line-of-sight (LOS). It can be shown that the frequency shift is equal to xcexdLOS/xcex, where xcexdLOS is the velocity of the relative movement between the satellite and receiver along the LOS, and xcex is the wavelength of the transmission. The Doppler shift is positive if the receiver and satellite are moving towards one another along the LOS, and is negative if the receiver and satellite are moving away from one another along the LOS.
The Doppler shift alters the perceived code phase of a satellite transmission from its actual value. Hence, the GPS receiver must correct the satellite transmissions for Doppler shift before it attempts to determine the code phase for the satellite through correlation analysis.
The situation is illustrated in FIG. 1, which shows a GPS receiver 10 and three GPS satellites 12a, 12b, and 12c. Each satellite 12a, 12b, 12c is transmitting to the GPS receiver 10. Satellite 12a is moving towards the GPS receiver 10 along the LOS at a velocity xcexda+14; satellite 12b is moving away from the GPS receiver 10 along the LOS at a velocity xcexdbxe2x88x9216; and satellite 12c is moving away from the GPS receiver 10 along the LOS at a velocity xcexdCxe2x88x9218. Consequently, assuming a carrier wavelength of xcex, the transmission from satellite 12a will experience a positive Doppler shift of xcexda+/ xcex; the transmission from satellite 12b will experience a negative Doppler shift of             v      b      -        λ    ;
and the transmission from satellite 12c will experience a negative Doppler shift of             v      c      -        λ    .
The GPS receiver functions by sampling the received signal 20 over a defined sampling window and then processing the samples. The duration of the sampling window is chosen to achieve a target signal to noise ratio (SNR). The target SNR is chosen to permit the presence and range of the satellites to be accurately detected. If the duration is too short, the signal may be such that there is no correlation value for a particular set of hypotheses which is significantly larger than the correlation values resulting from the other hypotheses tested. The duration of the sampling window must then be increased in order to increase the signal to noise ratio of the received signal 20, and permit the presence and range of satellites visible to the receiver to be accurately detected.
In addition to the biphase PN modulation of the GPS carrier, there is also a 50 Hz data modulation. This superimposed data modulation carries information about the satellite orbits. In order to navigate, the system must collect this data so that the locations of the satellites can be calculated as a function of time. This is a necessary piece of information for determining the range to the satellite from the PN code phase. At 50 Hz, the data causes unknown phase flips every 20 milliseconds or data epoch.
GPS receivers typically function by achieving synchronization with certain ones of the collection of GPS satellites, and then maintaining synchronization in a continuous tracking mode of operation. However, in certain applications, such as those involving low power consumption or inherently low C/No, as when operating inside of buildings, an intermittent or code tracking only mode of operation is employed in which a reduced tracking loop bandwidth is used to maintain loop SNR. For example, when C/No falls below 26-28 dB-Hz, data collection and carrier tracking are no longer possible, and GPS receivers change to the code track only mode in which is not possible to receive the 50 Hz data streams to derive bit sync.
In applications such as these, prior art receivers typically attempt to detect a signal of interest or a parameter of the signal of interest by multiplying the segment of samples by a hypothesis about the signal of interest, and then non-coherently integrating the resulting product values over the duration of the sampling window. Non-coherent integration is employed because the phase reversals at the data epochs are unknown, and phase inversions on opposite sides of an inversion point subtract. The result is a decrease in the signal voltage with integration time rather than an increase as desired. In a typical implementation, the magnitude of successive ones of the product values are added together, and phase information represented by the successive values is ignored. The result is a correlation value which is a measure of the degree of correlation between the segment of samples and the hypothesis.
The problem is that noise which may be and typically is reflected in the product values has a magnitude, and when the product values are added together, the magnitude of the noise which is present in each of the values accumulates incrementally in the final sum due to the non-coherent integration procedure. In other words, noise from successive intervals does not have a chance to cancel out the noise from previous intervals. Since the noise magnitudes for successive values add together, the cumulative effect of the noise in the final correlation value can be quite substantial.
Due to the cumulative effects of the noise, the duration of the sampling window has to be substantially increased to achieve a target SNR. The result is that the time required for the GPS receiver to achieve synchronization with satellites visible to it is dramatically increased. In addition, the power consumed by the receiver is also dramatically increased. The problem is particularly acute for applications involving integrating a GPS receiver with a mobile wireless handset. The consumption of excessive power by such a device drains battery power, and reduces the call-time available from the handset.
Consequently, there is a need for a signal detector which overcomes the disadvantages of the prior art. Similarly, there is a need for a GPS receiver which overcomes the disadvantages of the prior art.